1. Field
The present invention relates generally to data communication, and more particularly to novel and improved techniques for combining power control commands received in a wireless communication system.
2. Background
In a wireless communication system, a user with a terminal (e.g., a cellular phone) communicates with another user via transmissions on the forward and reverse links with one or more base stations. The forward link (or downlink) refers to transmission from the base station to the terminal, and the reverse link (or uplink) refers to transmission from the terminal to the base station. The forward and reverse links are typically allocated different frequency bands.
In a Code Division Multiple Access (CDMA) system, the reverse link transmissions from a number of active terminals may be received concurrently at each base station. Since these transmissions occur over a shared frequency band, the transmission from each active terminal acts as interference to the transmissions from the other active terminals. For each received terminal, the interference due to the total received power from the other transmitting terminals degrades this terminal""s received signal quality, which is typically quantified by a signal-to-total-noise-plus-interference ratio (SNR). Since a certain minimum SNR needs to be maintained for each active terminal to provide the desired level of performance, the total interference from all transmitting terminals is typically indicative of the total capacity of the reverse link.
To maximize the reverse link capacity, the transmit power of each active terminal is typically controlled by a respective first (inner) power control loop such that the signal quality of the reverse link transmission received at the base station from this terminal is maintained at a particular target SNR. This target SNR is often referred to as the power control setpoint (or simply, the setpoint). A second (outer) power control loop is typically employed to adjust the setpoint such that the desired level of performance is maintained. This level of performance is typically quantified by a particular frame, packet, block, or bit error rate (FER, PER, BLER, or BER, respectively). The reverse link power control mechanism thus attempts to reduce power consumption and interference while maintaining the desired link performance for the active terminals. This results in increased system capacity and reduced delays in serving users.
Many CDMA systems support soft handoff (or soft hand-over) on the reverse link whereby a data transmission from an active terminal may be concurrently received by multiple base stations. Reception of the reverse link transmission via multiple signal paths provides diversity against deleterious path effects such as fading and multipaths. Soft handoff may thus improve the quality and reliability of the reverse link transmission (e.g., higher received signal quality if the transmissions received by multiple base stations are combined, and lower probability of dropped calls).
While a terminal is in soft handoff with a set of base stations, an inner power control loop is typically maintained by each base station in the active set to direct the terminal to adjust its transmit power. Conventionally, each base station determines the received signal quality for the terminal (e.g., by processing a pilot transmitted by the terminal), derives power control commands based on the received signal quality, and transmits the power control commands to the terminal. Each power control command directs the terminal to adjust its transmit power either up or down by some amount. Since each base station typically receives the reverse link transmission at a different signal quality, the power control commands from the base stations are not necessarily the same.
Conventionally, the terminal receives the power control commands from the base stations in the active set and compares each received command against a particular threshold to detect whether it is an UP command for a transmit power increase or a DOWN command for a transmit power decrease. The terminal then conventionally applies the xe2x80x9cOR-of-the-downsxe2x80x9d rule for the power control commands detected in each power control period, and adjusts its transmit power downward if any of the detected command directs the terminal to decrease its transmit power. Using this rule, if the received power control commands from any of the base stations are totally unreliable, the terminal would essentially reduce its transmit power half of the time regardless of the commands received from the more reliable base stations. Since this effect is not desirable, a further requirement is often imposed such that if any base station""s received power control command is deemed unreliable, it is excluded for the purposes of deriving the final power control decision. One way to accomplish this is by comparing each base station""s received signal strength (e.g., pilot power) against a xe2x80x9cpower lockxe2x80x9d threshold and discarding the received power control commands from each base station having received signal strength that falls below this threshold.
The conventional technique for combining power control commands received from multiple base stations is sub-optimal for several reasons. First, the terminal may erroneously receive DOWN commands from a marginally reliable base station that passes the power lock threshold, and these erroneous commands would then cause the final power control decision to be DOWN regardless of the other received commands from more reliable base stations. Second, incremental information from weak base stations that failed the power lock threshold is discarded and not used to derive the power control decision.
As can be seen, techniques that can be used to more xe2x80x9coptimallyxe2x80x9d combine received power control commands to improve reliability and system performance are highly desirable.
Aspects of the invention provide techniques to more effectively combine power control commands received from multiple active base stations. In an aspect, xe2x80x9csoft-decisionxe2x80x9d power control symbols for a number of active base stations in each power control period are combined to provide a single power control decision having improved quality. Each soft-decision power control symbol is a multi-bit value that is representative of a transmitted hard-decision (i.e., binary) power control command that has been distorted by channel and processing noise. In another aspect, the soft-decision power control symbols for the active base stations are scaled by their associated scaling factors prior to being combined. The scaling factor for each base station is related to the received signal quality for the base station, and the scaling allows power control symbols from better-received base stations to be given greater weights.
A specific embodiment of the invention provides a method for deriving power control decisions (e.g., for a reverse link transmission) in a wireless (e.g., CDMA) communication system. In accordance with the method, a received signal is initially processed to derive soft-decision symbols for power control commands transmitted from a number of transmission sources (e.g., base stations). Each soft-decision symbol for each base station is then scaled based on a scaling factor associated with the base station. The scaled soft-decision symbols for each power control period are then combined to provide a (decision) metric for the period. Each decision metric is then compared against a particular threshold, and a power control decision is derived for each decision metric based on the result of the comparison.
The soft-decision power control symbol combining techniques described herein may be used for various wireless communication systems (e.g., IS-95, cdma2000 and W-CDMA systems). These techniques may also be advantageously used in the forward and/or reverse links. Various aspects, embodiments, and features of the invention are described in further detail below.